1. Field of the Invention
This invention relates generally to optical pellicles, and more particularly, to monolithic hard pellicles for use in photolithography and methods of forming the same.
2. Description of Related Art
Pellicles have played an important role in the fabrication of semiconductor circuits. A conventional pellicle is a thin, optically transparent membrane stretched over a rigid metal frame for attachment to a photomask via such frame. In so doing, the pellicle is attached to the photomask such that the membrane is positioned at a distance spaced away from the photomask. The pellicle is then used in photolithography to protect the patterned photomask surfaces from contamination by airborne particles. That is, smaller particles and dust falling onto the pellicle plate is out of focus during the photolithography process, and will not therefore be reproduced in the silicon wafer.
To fabricate a pellicle, a very thin membrane, normally in the range of 0.5 to 3.0 microns and usually made of organic or fluorocarbon polymers, is first formed on a substrate. The membrane is then removed and mounted to the rigid frame. These pellicle frames take many shapes, depending upon the photolithography equipment with which the pellicle is ultimately to be used. The frame always encloses the entire printable area of the mask so that the pellicle it supports is able to keep particles off this critical area.
An emerging photolithography technique is the use of radiation having a 157 nm (nanometer) wavelength to expose the desired circuit pattern from the photomask to the resist layer on the wafer substrate. Extension of optical lithography to 157 nm has received widespread support as it offers the prospect of improved resolution based on decades of optical lithography experience. However, the development of pellicles for use in 157 nm lithography has been challenging due to the high energy of the photons, such as those at energies of about 7.9 eV. This is because most common lithographic materials available for the fabrication of thin (0.5 μm to 2.0 μm) protective pellicles used at 365 nm, 248 nm, and 193 nm exposure wavelengths are either not transparent enough and strongly absorb at the 157 nm wavelength; or are not durable enough to withstand more than a few minutes of exposure without severe degradation at the 157 nm exposure wavelength.
Since no polymers have been found with sufficient radiation durability to be used as pellicles to protect photomasks from contamination at the 157 nm exposure wavelength, thick or hard quartz (crystallized silica) plate pellicles continue to be used at the 157 nm wavelength. The hard pellicles for use at such wavelength are conventionally made by attaching, such as by bonding, sealing, encapsulating, and the like, modified fused silica plates to fused silica frames. The pellicle and frame unit is then mounted onto photomask substrates via the fused silica frames to protect the patterned photomask surfaces from contamination by airborne particles.
This conventional process of forming pellicles by attaching a frame to a fused silica plate typically results in small distortions or stresses that can have various effects on the pellicle itself as well as the photomask being protected by such pellicle. For instance, when distortions or stresses occur as a result of attaching a frame to a mask substrate to form a hard pellicle, these distortions and/or stresses are transferred to the hard pellicle, which in turn, create optical distortions or aberrations that distort the projected light during wafer exposure. Further, such stresses may undesirably distort the reticle or even cause stress-induced birefringence or double refraction, i.e., splitting of a light wave into two unequally reflected or transmitted waves, which may result in problems with wafer printing.
As such, conventional processes and the resultant thick and/or hard pellicles formed by such processes are not suitable for efficiently, effectively and reliably protecting photomasks from contamination at the 157 nm exposure wavelength. Consequently, there is a need for improved methods for forming pellicles for use at the 157 nm exposure wavelength.